That type of turbomachine including a short diffuser between the injectors and the flame-catchers is characterized by a small bypass ratio seen from the rear body. The secondary flow serves in particular to cool the parts situated downstream from the injectors and must be used effectively for this purpose.
Conversely, the primary flow coming from the low pressure turbine is flowing at a high rate. It is on this flow that the performance of the engine relies for the most part. Consequently, it must be subjected to a minimum amount of head losses and must be as uniform as possible in temperature and speed. For this purpose, the afterburner chamber diffuser constituted by the confluence sheet serves to slow down the primary flow upstream from the flame-catchers and to channel it so that, downstream, it occupies the entire volume of the afterburner chamber. This function, which is known as a “diffusion” function since it is accompanied by an increase in static pressure, needs to take place without unwanted turbulence forming along the stream, since such turbulence or backflow leads to losses that can cause the fuel coming from the upstream injectors to self-ignite.
In afterburner chambers having a large dilution ratio, the primary flow and a portion of the secondary flow meet and mix. Conversely, when the dilution ratio is small, the fraction of the secondary flow available for mixing at the downstream end of the confluence sheet is small once all of the flows needed for ventilation have been taken off. The confluence sheet therefore needs to be flared so that the primary flow occupies the full extent of the afterburner chamber. If the sheet is not properly shaped, backflow occurs in the vicinity of the confluence sheet between the plane of the injectors and the plane of the flame-catchers. This backflow is particularly encouraged when the amount of gyration at the outlet form the low pressure turbine is large.
A risk of backflow thus appears in the vicinity of a highly diffusing confluence sheet associated with a large amount of gyration in the primary flow. This backflow is predicted by numerical 3D simulation of the flow. It appears in the primary flow at the concave bend of the confluence sheet, where the sheet provides an indentation suitable for a pocket of backflow.
Furthermore, at the junction between the convex bend portion and the concave bend portion of the confluence sheet temperature and temperature gradients are excessive. Steep temperature gradients are due to the convection of the cold secondary flow which blows against the outside surface of the sheet and the hot primary flow which blows against its inside surface.
In order to eliminate turbulence, it would naturally be possible to modify the shape of the confluence sheet by lengthening the diffuser axially, but that solution increases the size of the engine.